In accompany with the advancement of multimedia technology in recent years, a large quantity of information including a picture data is required to be processed. Moreover, a larger capacity of a storage unit for recording the information is required. In particular, in the storage field of the video data of a high image quality, a more storage capacity than a current DVD (Digital Versatile Disc) is desired. However, in order to increase the storage capacity of an optical disc apparatus or hard disc apparatus, a record density is required to be increased. In association with this necessity, the reduction in an error rate and the reservation of reliability become the important subjects. For such subjects, the optical disc is roughly considered in the direction of three kinds such as a medium composition approach, an optical approach and a signal processing approach. Mainly, the signal processing approach will be described below.
An optical disc apparatus detects information from brightness or polarization of a reflection light by emitting a laser beam collected by optical elements onto a disc medium. The collected beam spot is definite. As its diameter is smaller, the recording/reproducing of a higher density is possible. For this reason, the optical approach for decreasing the beam spot is advanced. The spot diameter is inversely proportional to a natural aperture NA of an objective lens and proportional to a laser beam wavelength ●. Thus, when the natural number NA is increased and the wavelength ● is decreased, the spot diameter can be made small. However, when the natural number NA is increased, a focal depth is made shallow, and a distance between a disc plane and a lens is required to be made short, which results in a limit. Also, the short wavelength laser has the subjects of the stability of a high output, oscillation, a long life and the like. However, the shorter wavelength is gradually advanced, such as an infrared laser (●=780 nm) in CD (Compact Disc), a red laser (●=650 nm) in DVD, and a blue laser (●=405 nm) in a next generation DVD.
A transmission path frequency property between an optical head and the disc medium exhibits the shape of a LPF (Low-Pass Filter) in which the definite beam spot causes reduction of gain in a high frequency region. Thus, even if a rectangular wave is recorded, its waveform is made dull. If the record density is made high, the waveform to be read at a particular time interferes with the waveform of a different time. This is referred to as an inter-code interference. With the inter-code interference, it becomes difficult to reproduce a record mark shorter than a certain length. On the contrary, if the record mark is long, a reduction in the phase data output, frequency for synchronization clock extraction causes it to be out of synchronization. Thus, the record mark is required to be limited to the certain length.
From the above reasons, as approach of signal processing, data to be recorded onto the optical disc is coded. In particular, an RLL code (Run Length Limited Code) in which an inversion distance of a code is limited is used in many cases. An ETM (Eight to Twelve Modulation) code, an EFM (Eight to Fourteen Modulation) code, a (1,7) RLL code, an 8/16 modulation code and the like are used. Among them, the shortest run length in the EFM code used in CD and the 8/16 modulation code employed in DVD is 2 (d=2). The shortest, run length in the (1,7) RLL and ETM codes is 1. The ETM is (1,1,0) RLL code, as described in “Eight to Twelve Modulation Code for High Density Optical Disk” (International Symposium on Optical Memory 2003, Technical Digest pp. 160-161, Nov. 3, 2003) by Kinji Kayanuma, et al. The coding rate of the ETM code is 2/3 similarly to the (1,7) RLL code. There are the features in the limit of the continuous number of the shortest, marks and the DC component compression performance.
Also, there is a technique referred to as waveform equalization. This technique reduces an error rate by inserting an inversion filter for removing the inter-code interference. The waveform equalization can suppress the inter-code interference, by emphasizing a high frequency component of a readout signal. However, this technique also emphasizes a high frequency component of noise. Thus, there is a case that the waveform equalization degrades SNR (Signal to Noise Ratio) of the readout signal. In particular, when a record density is high, the degradation in the SNR due to the waveform equalization results in a main factor of error in data detection. A PR (Partial Response) equalization is one method of the waveform equalization that intentionally generates a known inter-code interference. The PR equalization can suppress the degradation in SNR, because the high frequency component is not usually emphasized.
On the other hand, as an effective detecting method, there is a maximum likelihood detecting system. This method is a method for improving the detection performance by selecting a pattern, in which a root mean square value of errors is the smallest, from all of possible time-series patterns in which it is known that a state transition occurs. However, it is difficult to perform the above process on an actual circuit, because of a circuit scale and an operation speed. Thus, this is usually attained by using an algorism referred to as a Viterbi algorism and recurrently selecting a path. This detecting method is referred to as the Viterbi detection.
A detecting method in which the Viterbi detection is combined with the PR equalization is referred to as a PRML (Partial Response Maximum Likelihood) method, and while a kind of error correction is carried out, data can be detected. In the PR equalisation, a readout signal has a correlation in a temporal direction. For this reason, only a particular state transition appears in a data sequence obtained by sampling the readout signal. It is possible to reduce the error of the detection data by comparing a limited state transition and the data sequence of the actual readout signal including noise and selecting the most probable state transition. The PRML detection method that uses the ETM code and a PR (1,2,2,2,1) channel is described in “Development of HD DVD Apparatus Technique (Recording Technique)” (Information Media Academy Technique Report ITS Technical Report Vol. 28, No. 43, PP. 17-20 MMS2004-38, CE2004-39 (July 2004)) by Ogawa, Honma et al. According to this technique, it is possible to attain a wide detection margin at a time of a high density recording/reproducing
In order to improve the detection performance by performing the Viterbi detection, the frequency characteristic of a reproduction channel needs to coincide with a particular PR equalisation characteristic. In such a case, the PR equalization characteristic as close as possible to the reproduction channel is selected. However, the frequency characteristic is typically corrected by a waveform equalizer and made as equal as possible to a predetermined PR characteristic.
As a technique for adaptively correcting aging degradation in a signal and improving the detection performance, there is an automatic equalization or an adaptive equalization. Although adaptive equalisation algorism of a sequential type is described in “Base of Today's Information Communication” (Ohmsha Ltd, Dec. 20, 1992 pp. 212-217) by Shuzo Saito et al. In particularly, a Zero Forcing method, a Mean Square method and the like are typical. The adaptive equalization technique has great effect making it possible not to necessitate of initial adjustment of the apparatus.
Since an amplitude data is used in the PRML, an AD converter having the data width of 8 bits or so on and operating in a channel clock is required. Also, when an equalizer for equalization to the PR channel is formed from an FIR (Finite Impulse Response) filter, the FIR filter needs to have about ten stages of multipliers and an adder. Moreover, a tap coefficient controller is required to contain correlation units for tap coefficients of the FIR filter.
However, the operation speed of a reproducing circuit system is made faster in accordance with a request of a high magnification speed reproduction. In order to improve an operation speed of the AD converter, the circuit scale becomes large, and a power consumption amount simultaneously increases. Also, in order to operate the FIR filter faster, the scale of the multiplier circuit becomes large, and the power consumption amount also increases. Moreover, in order to operate the tap coefficient controller fast, the circuit scales of a multiplier and an integrator in the correlation unit become large, and their power consumption amounts also increase.
Thus, the high magnification speed reproduction involves the increase in the circuit scale and the power consumption amount. Occasionally, a case that the Viterbi detecting section cannot operate is considered. This results in a serious demerit in case of installation in note type personal computer or the like that is used under a battery drive.
One method to solve the foregoing problems is disclosed in Japanese Patent No. 3,688,225. FIG. 1 is a block diagram showing a configuration of a digital data readout apparatus. The digital data readout apparatus of this type contains a pickup 101, a preamplifier 102, a waveform equalizer 103, an AD converter (Analog-to-Digital Converter) 104, an offset control section 105 for half rate processing, a phase error data detecting section 108 for half rate processing, a loop filter 109, a clock generating section 110, an adaptive equalizing section 106 for half rate processing and a maximum likelihood decoding section 107 for half rate processing.
An optical recording medium 100 digitally records record data with record code that has a constraint in which there are at least three or more continuous same codes (d≧2). In the digital data readout apparatus, at first, a readout signal is obtained by performing sampling by the AD converter 104 which uses a sampling clock signal having a frequency equal to a half of a channel bit frequency from the optical recording medium 100. The half rate processing offset control section 105 carries out an offset correction control while interpolating the readout signal in a default time. The half rate processing phase error data detecting section 108 performs a phase synchronization control on the readout signal obtained by the half rate processing offset control section 105, while interpolating a phase error data in the default time. Simultaneously, the half rate processing adaptive equalizing section 106 performs a partial response adaption equalisation on the readout signal obtained by the half rate processing offset control section 105. Then, the half rate processing maximum likelihood decoding section 107 uses the partial response adaptive equalization signal obtained by the half rate processing adaptive equalizing section 106 and carries out the decoding on the basis of a type of partial response.
Specifically, a circuit system can operate at a frequency equal to a half of the conventional case, and the power consumption amount can be largely decreased without any reduction in reproduction performance. However, this method can be used for only CD and DVD in which the shortest run length of the record code is 2. When the code whose shortest run length is 1 is used, the frequency band of the readout signal is extended up to a high frequency region. Thus, when the sampling is carried out at the half rate, the necessary band cannot be reserved. Therefore, the influence of aliasing leads to the reproduction performance reduction. The code whose shortest run length is 1 is used in the next generation optical disc of the DVD in which a blue laser is used. Also, there is a standard in which the PRML detection is assumed, and the channel rate is very high.
Also, Japanese Patent Application Publication (JP-P2001-101799A) discloses a digital readout signal processing apparatus for determining a digital record data from an analog readout signal that is read from a recording medium by a reading head. The digital readout signal processing apparatus contains an AD converter, an equalizing circuit, an interpolator and a determining unit. The AD converter samples the analog readout signal in a period longer than a digital record channel rate at least and converts into a low rate digital readout signal of a period longer than the digital record channel rate. The equalizing circuit digitally filters the low rate digital readout signal at an original low rate and generates a digital equalization signal. The interpolator interpolates reproduction data of the digital record channel rate from the digital equalization signal. The determining unit derives the recorded data from the interpolated data column. The sampling in the AD converter is carried out at a half rate that is a frequency equal to a half of the record channel rate. The interpolator carries out a half rate Nyquist interpolation whose band is limited to the Nyquist frequency of the half rate.